Known systems for alignment of wheels of motor vehicles employ computer-aided, three-dimensional machine vision alignment apparatus. Examples of so-called “3D alignment” systems are described in commonly assigned U.S. Pat. No. 5,724,743 to Jackson, entitled “Method and apparatus for determining the alignment of motor vehicle wheels,” and commonly assigned U.S. Pat. No. 5,535,522 to Jackson, entitled “Method and apparatus for determining the alignment of motor vehicle wheels,” both of which are commonly assigned to the assignee of the present application and incorporated herein for reference in their entireties. Sensors, such as cameras, view alignment targets affixed to the wheels in a known positional relationship to determine the position of the alignment targets relative to the alignment cameras. The alignment cameras capture images of the targets. From these images, the spatial orientation of the wheels can be determined and related to specification alignment characteristics. Wheel alignment adjustments can be made, while maintaining camera sensing, until the captured images correspond to specification values.
More recent versions of 3D alignment systems favor using identifiable wheel features to determine the spatial orientation of a wheel, in lieu of attaching targets to the wheels. With such operation, the cost of targets, a variety of attachment devices needed for different vehicle models, and the technician labor involved in the proper attachment of the targets, is eliminated. A wheel characteristic that can be sensed to derive the necessary spatial relationships, for example, may be the boundary between tire and wheel, or simple targets attached to the wheel.
These machine vision wheel alignment systems utilize measurement modules that need an unobstructed side view of the vehicle wheels. Computer software processes the images to distinguish the wheel from the tire and calculate wheel alignment parameters based on the extracted image of the wheel. The measurement modules typically include one or more digital cameras, illumination devices for illuminating the wheel during image capture, and a structure for supporting these various devices. A typical wheel alignment system may have one measurement module for each wheel of the vehicle being measured. To optimize measurement performance the distance from the wheel to the measurement module must be predetermined and allowed to vary over a fairly narrow range. It is advantageous to have the measurement module in a position such that it is longitudinally aligned with the center of the wheel it is measuring.
Systems that have been developed to date typically use self-standing measurement modules that contain the actual measuring devices. The measurement modules are installed at a fixed height to the side of the vehicle lift and can be in the way of the shop personnel when they are not being used for a wheel alignment. During a wheel alignment measurement, the vehicle is usually placed on a vehicle lift. Normally the operator will raise or lower the lift during the alignment process to make adjustment of the vehicle easier. The cameras require very wide angle lenses to be able to capture wheel images for vehicles with various wheelbases and for vehicles that are moved relative to stationary measurement modules. The wheel alignment process also may require sensing while the vehicle is moved longitudinally when the lift is in the lowered position.